Specific Heat of Liquid ${\mathrm{He}}^3$/${\mathrm{He}}^4$ Mixtures near the Junction of the $\lambda$ and Phase-Separation Curves. I

The specific heat of liquid ${\mathrm{He}}^3$/${\mathrm{He}}^4$ mixtures has been measured at saturated vapor pressure for eight values of the ${\mathrm{He}}^3$ mole fraction $x$ in the range $0.53\le x\le 0.73$ at temperatures from ∼0.6 to ∼1.4°K. For purposes of comparison, the specific heat of pure liquid ${\mathrm{He}}^3$ has also been measured under similar conditions. The measured specific heat, obtained with a calorimeter almost completely full of liquid, has been corrected to yield the specific heat of the liquid $c_{P,x}$ at constant pressure and mole fraction. For the mixtures, the correction required was everywhere ≲ 1%. To a resolution in temperature which varies from 0.1 to 3 m°K from one value of $x$ to another, the specific heat at the $\lambda$ peak appears to be finite, continuous, and cusped. Both the $\lambda$ peak and the discontinuity in $c_{P,x}$ as the system goes from the two-liquid-phase region to the one-liquid-phase region tend to disappear as the junction of phase boundaries is approached.